In the case of motor vehicles with internal combustion engines, ever stricter exhaust emissions limits mean that air pollutants, such as nitrogen oxides (NOx), in the exhaust gas flow must be reduced to a greater and greater extent. One known method which is used in this context is catalytic reduction (known as “Selective Catalytic Reduction” or SCR). In this case, a reductant is pumped out of a tank as far as a metering module in the region of the exhaust line by means of a pump. The metering module required to inject the reductant is arranged within the exhaust line, generally ahead of the catalyst in which the reduction of nitrogen oxides takes place. A 32.5% aqueous urea solution (known as “AdBlue®”) is generally used as a reductant. In many cases, diaphragm pumps are used to deliver the reductant, and these generally have a preferred direction of delivery.
In many cases, the diaphragm pumps are driven by means of an electric motor having an eccentric connected by a connecting rod to the pump diaphragm. If the eccentric is set in rotation with the aid of the electric motor, the pump diaphragm is periodically raised and lowered by the connecting rod, with the result that the reductant is drawn in from the storage tank and pumped as far as the metering module. An orifice or restrictor arranged downstream of the diaphragm pump prevents a pressure rise in the system when the metering module is closed or is delivering only a very small quantity of the reductant into the exhaust line. For this purpose, the delivered quantity that is not required is directed back into the storage tank by the orifice via an additional return line. The exhaust gas aftertreatment system furthermore has a complex open-loop and/or closed-loop control device for controlling all the system processes and a large number of sensors and actuators, which are interconnected via a bidirectional bus system.
The aqueous urea solution freezes below −11° C. In order to ensure the required ice pressure resistance after the internal combustion engine is switched off, the reductant must as far as possible be sucked back completely out of all regions which are exposed over a prolonged period to a temperature of −11° C. or less. In order to be able to implement the suck-back process by means of the diaphragm pumps which are conventionally employed, a separate 4/2-way valve is generally used.
In normal delivery mode, in particular during the normal operation of the internal combustion engine, the 4/2-way valve is in the deenergized idle state, allowing the reductant to be pumped out of the tank with the aid of the diaphragm pump and reach the metering module via the 4/2-way valve. While the reductant is being sucked back out of the exhaust gas aftertreatment system, the delivery direction of the diaphragm pump can be maintained unaltered. Only the 4/2-way valve is activated. In normal delivery mode, the reductant flows in opposite directions through two parallel ducts in the 4/2-way valve while, in suck-back mode, the reductant flows in opposite directions through two further, intersecting ducts in the 4/2-way valve.
However, multi-way valves of this kind involve a construction of complex design and are therefore also expensive to produce. Moreover, such valves are prone to leaks, are susceptible to wear and require a lot of maintenance.